Currently, the GNOME build is failing due to no support for riscv:

checking for a supported CPU... no (riscv64)
configure: error: Unsupported host architecture. Sorry

There is a forked valgrind with experimental riscv support but I don't know if this is really ready to use:

https://github.com/petrpavlu/valgrind-riscv64

It should still be possible to build GNOME by making the following changes:

global USE: -valgrind
libinput: -test
x11-libs/amtk-5.0.0: patch or USE tweak to disable valgrind (unconditional dep, currently. My guess is that it can actually be turned off.)

This should allow the gnome stage3 to get past this roadblock, which is currently preventing it from building. I haven't looked into what is pulling it in but it is most likely amtk-5 that is causing the issue, since I don't think valgrind or test are set in USE by default. (This would explain why gnome doesn't build now, as it built before on riscv. But we recently tweaked amtk if I recall correctly.)